mfd: madera: Fix bad reference to pinctrl.txt file

The pinctrl.txt file was converted into reStructuredText and moved into
driver-api folder. This patch updates the broken reference.

Fixes: 5a9b7383 ("pinctrl.txt: move it to the driver-api book")
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>